Paper making machines employ endless belt-type screens upon which the paper is deposited. Normally, the paper is deposited in a substantially uniform layer in the center of the screen and the marginal regions on either side, up to about 50 cms wide, do not carry paper. The edge regions of the screen may be in abrading contact with the upper edges of suction boxes and the like which are used to withdraw water from the paper material or for other purposes.
From German application (AS) No. 1,022,089 it has been known to artifically elongate the marginal region of a screen relative to the center thereof and to set the screen fabric in this state. Although this measure increases the lifetime of a screen for a papermaking machine, the lifetime is nevertheless limited by the greater wear on the marginal regions of the screen.
From German application (OS) No. 1,561,679 it has been known to increase the lifetime of a screen for a papermaking machine by selecting a material of higher wear resistance for the longitudinal threads in the marginal regions. However, this publication exclusively relates to screens made from metal alloys.
Consequently, conventional papermachine screens are subject to higher wear in a marginal region of about 50 cm width than in the central region used for papermaking which will be briefly referred to as paper region. Furthermore, there is especially high wear at the format confining strips. The term "marginal region" as used herein shall include also the region of wear caused by the format confining strips and, in general, all regions subject to especially high wear. The exact cause for this greater wear of the marginal regions is unknown. However, it seems to be significant that the marginal regions run outside the suction box openings or on top of the margin of the suction box openings. Attempts have been made to prevent the higher wear of the marginal regions, for instance, by designing the openings of the suction boxes such that the lateral confinements extend obliquely with respect to the travel of the paper machine screen. However, these measures have been only partially successful.